The present invention relates to wireless communication systems and more specifically to a system for improved communication between a base station, a plurality of mobile units and a plurality of portable units.
Wireless communication systems conventionally comprise a plurality of base stations, a plurality of mobile units, and a plurality of portable units. For purposes of the present disclosure, a xe2x80x9cmobile unitxe2x80x9d refers to a communication device located within a vehicle and powered by a battery located in the vehicle. Conventionally, a mobile unit is capable of 20 Watt RF transmission power. For purposes of the present disclosure, a xe2x80x9cportable unitxe2x80x9d refers to a communication device located on a person powered by a smaller portable battery that is typically unitary with the communication device. Conventionally, a portable unit is capable of 3 Watt RF transmission power. Each base station is fixed at a single geographic location and transmits high powered RF energy from an antenna mounted at a significant height above ground level. Similarly, the base station receives RF energy at an antenna mounted at a similar significant height above ground level. Each base station communicates with one or more portable units that are geographically positioned within some radius defined by minimum power level necessary to successfully receive an RF signal transmitted to and from the base station. The base stations typically communicate with a central office over a land based cable network or microwave point to point using a frequency range different from the base station to mobile unit communication. The central office performs most of the switching and routing functions, which allows the multiple base stations to inter-operate as a single wireless system.
Portable units are typically hand held by the wireless system user and operate to communicate with one of the base stations to provide connectivity to the larger wireless system. Ideally, a user with one of the portable units is able to maintain communication anywhere within a defined geographic reach. An important metric of the quality of a wireless communication system is the percentage of coverage available within a given geographic area. One of the practical realities of a wireless system, however, is that natural and artificial topographies create communication xe2x80x9cshadowsxe2x80x9d where communication between a portable unit and a base station will degrade or cease altogether. A mobile unit, however, may still be within the coverage area supported by the base station as a result of its higher powered RF transmission capacity. The size and number of the shadows reduce the percentage of portable coverage within a given geographic area.
One solution to the shadow problem is to erect an additional base station that is able to transmit and receive within the shadowed area. Additional base stations as with any increase in infrastructure, however, significantly increase the cost of the overall system. Often the shadowed area is infrequently used and the amount of use does not warrant the cost and maintenance of an additional base station.
Another solution to the shadow problem is to add a component to the overall wireless system called a mobile vehicular repeater unit. The mobile vehicular repeater unit operates as a movable base station that is permanently mounted in a vehicle driven by a user of a portable unit. The mobile vehicular repeater unit acts as an intermediary between the portable unit and the base station to relay information received from the portable unit to the base station and vice versa. The mobile vehicular repeater unit, in practical effect therefore, becomes a xe2x80x9croaming base stationxe2x80x9d by receiving communications from the base station and further transmitting those communications to one or more portable units. Prior art mobile vehicular repeater solutions implement communication between the portable and the mobile vehicular repeater unit in what is known in the industry as xe2x80x9ctalk around modexe2x80x9d. In xe2x80x9ctalk around modexe2x80x9d, the mobile unit and the portable unit employ simplex communication in a frequency band that the base station uses to transmit a signal to the portable unit. Advantageously, use of the vehicular repeater permits an increase in portable unit coverage. The coverage, however, is over a simplex channel, which decreases the available bandwidth of communication. It would be desireable to increase the portable unit coverage with duplex communication without increasing costly infrastructure.
The vehicular repeater mobile vehicular repeater unit solution is desirable because the less used shadowed area is illuminated temporarily and only when communication to the shadowed area is needed. The mobile vehicular repeater unit solution is reliable because the mobile vehicular repeater unit is typically positioned closer to the user than the base station and has higher transmission power to the base station and higher gain reception capability than typical portable units. Advantageously, the mobile vehicular repeater unit is less costly than a base station.
One of the disadvantages to the mobile vehicular repeater unit solution previously described is that RF signals transmitted by the mobile vehicular repeater unit to the portable unit can interfere with signals received by the mobile vehicular repeater unit from the base station. In a full or half-duplex wireless communication system, each radio channel consists of two frequencies which are separated by a fixed frequency offset. The Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) has defined the forward channel portion, i.e. base station transmission to the portable unit/mobile unit, as operating at the higher frequency of the pair. The FCC has further defined the reverse channel portion, i.e. base station reception from the portable unit/mobile unit, as operating at the lower frequency of the pair. Given this convention, the mobile vehicular repeater unit is receiving from the base station in the same band as it is transmitting to the portable unit. Disadvantageously, the mobile vehicular repeater unit transmitter can desensitize its own receiver. A solution to the desensitization of the receiver by the transmitter is typically expensive frequency management, frequency separation, and complicated RF design. There is a need, therefore, for a reliable and cost effective solution to the disadvantages of the mobile vehicular repeater unit implementation.
An object of a communication system according to the teachings of the present invention is to increase the percentage portable coverage while minimizing the amount of infrastructure necessary to provide the increased coverage.
A communication system comprises a base station, a mobile vehicular repeater unit, and a portable unit. The base station transmits to the mobile vehicular repeater unit over an upper frequency channel portion. The mobile vehicular repeater unit transmits to the base station over a lower frequency channel portion. The mobile vehicular repeater unit transmits to the portable unit over the lower frequency channel portion. The portable unit transmits to the mobile vehicular repeater unit over the upper frequency channel portion.
A method for communicating in a communication system, the communication system comprises at least one base station, a primary mobile vehicular repeater unit, a primary portable unit and at least one alternative mobile vehicular repeater unit. The method comprises the steps of the portable following a frequency use plan of transmitting on a lower frequency channel portion and receiving on an upper frequency channel portion. The process further comprises attempting to establish communication between the primary portable unit and any one of the at least one base stations. If the attempt to establish communication between the primary portable unit and any one of the at least one base stations fails, the method modifies the frequency use plan for the primary portable unit by transmitting on the upper frequency channel portion and receiving on the lower frequency channel portion. The method then comprises the steps of attempting to establish communication between the primary portable unit and any one of the at least one alternative mobile vehicular repeater units. If the attempt to establish communication between the primary portable unit and any one of the at least one alternative mobile vehicular repeater units fails, the method establishes communication between the primary portable unit and the primary mobile vehicular repeater unit and between the primary mobile vehicular repeater unit and any one of the base stations.